The early history of modern birds inferred from DNA sequences of nuclear and mitochondrial ribosomal genes

The traditional view of avian evolution places ratites and tinamous at the base of the phylogenetic tree of modern birds (Neornithes). In contrast, mo st recent molecular studies suggest that neognathous perching birds (Passer iformes) compose the oldest lineage of modern birds. Here, we report signif icant molecular support for the traditional view of neognath monophyly base d on sequence analyses of nuclear and mitochondrial DNA (4.4 kb) from every modern avian order. Phylogenetic analyses further show that the ducks and gallinaceous birds are each other's closest relatives and together form the basal lineage of neognathous birds. To investigate why other molecular stu dies sampling fewer orders have reached different conclusions regarding neo gnath monophyly, we performed jackknife analyses on our mitochondrial data. Those analyses indicated taxon-sampling effects when basal galloan-serine birds were included in combination with sparse taxon sampling. Our phylogen etic results suggest that the earliest neornithines were heavy-bodied, grou nd-dwelling, nonmarine birds. This inference, coupled with a fossil bias to ward marine environments, provides a possible explanation for the large gap in the early fossil record of birds.